Pointing devices, such as computer mice or trackballs, are well known for inputting data into and interfacing with personal computers and workstations. Such devices allow rapid relocation of a cursor on a monitor, and are useful in many text, database and graphical programs. A user controls the cursor, for example, by moving the mouse over a surface to move the cursor in a direction and over distance proportional to the movement of the mouse. Alternatively, movement of the hand over a stationary device may be used for the same purpose.
Computer mice come in both optical and mechanical versions. Mechanical mice typically use a rotating ball to detect motion, and a pair of shaft encoders in contact with the ball to produce a digital signal used by the computer to move the cursor. One problem with mechanical mice is that they are prone to inaccuracy and malfunction after sustained use due to dirt accumulation, etc. In addition, the movement and resultant wear of the mechanical elements, particularly the shaft encoders, necessarily limit the useful life of the device.
One solution to the above problems has been the development of optical mice. Optical mice have become very popular, because they provide better pointing accuracy. One approach for measuring linear displacements uses an array of photo-sensors or detectors, such as photodiodes, in which the output of the individual detectors in the array are wired together in a repeating pattern spanning two or more detectors, referred to here as the M value of the array. Generally, the distribution of spatial frequencies in an image on the array is roughly centered around the spatial frequencies of the detectors, and signal processing techniques are used to track the movement of this image back and forth across the array.
The dominant architecture used in previous embodiments used two or more one dimensional (1D) arrays at substantially orthogonal angles to one another to track motion in two directions or dimensions. The drawback of this approach includes limited accuracy along directions that deviate significantly from the detector-array orientations. This is due to the fact that an off-axis image enters and leaves the array too quickly before the image has a chance to build-up an unambiguous signal. Thus, this approach does not provide sufficient redundancy for low-noise operation. In particular, for linear comb arrays having axes oriented orthogonally, motion primarily along one axis leads to a small signal in the orthogonal axis, and hence a small signal to noise ratio along the orthogonal axis.
An alternative implementation uses correlation uses a square two dimensional (2D) array consisting of pixel architectures that take differences along orthogonal axes, and therefore also do not provide needed redundancy in the outputs.
Accordingly, there is a need for an optical pointing device that is capable of detecting movement with an acceptable path error in both on-axis and off-axis directions. It is further desirable that the array have an architecture that simplifies data processing requirements.